1. Field of the Invention
This invention relates to the dissipation of heat from a circuit board having bare silicon chips mounted thereon.
2. Description of Related Art
Circuit boards having mounted thereon electronic components such as integrated circuit chips are of course well known. Frequently the components generate sufficient heat so that a heat dissipation means must be provided. One means is simply a fan blowing air across the circuit board. Another means is a heat sink thermally connected to the circuit board (or the components). The heat sink may be positioned either on the front side of the circuit board (the side bearing the components) or the back side (the side opposite the one bearing the components). A thermally conductive material may be used to establish thermal contact between the heat sink and the heat-producing components and/or the circuit board. Illustrative disclosures relating to heat dissipation in circuit boards include Cipolla et al., U.S. Pat. No. 5,268,815 (1993); Kim et al., U.S. Pat. No. 5,552,635 (1996); and Shuff, U.S. Pat. No. 5,812,374 (1998).
There have been recent developments involving circuit boards in which the components are bare silicon chips. Bare silicon chips are chips in which the exposed surface is silicon unprotected by molded plastic packaging (although the chip may have a thin passivating or protective layer). Bare silicon chip technology is also known as Direct Chip Attach or DCA technology and is further described in publications such as Electronic Packaging & Production, pp. 12-20 (NEPCON West '99). An example is DRAM chip sets developed by Rambus Inc., of Mountain View, Calif. Because these chips consume more power than a typical memory chip, a more aggressive heat dissipation design is required. For example, if a fan alone is used for heat dissipation, a ducted fan with extremely high flow rate is required, at a cost of increased power consumption and noisiness. In other designs, two fans are used, one for cooling the microprocessor chip and a separate, dedicated one for the DRAM chips. At the same time, unpackaged nature of bare silicon chips makes them more vulnerable to damage if proper care is not taken.
Raychem Corporation, of Menlo Park, Calif., has sold internally supported thermally conductive gel materials as interface materials for heat dissipation in circuit boards. Such materials are depicted in FIGS. 1a and 1b of allowed copending, commonly assigned application of Mercer et al., Ser. No. 08/746,024, filed Nov. 5, 1996. A customer purchases the supported gel material and affixes it to his own circuit board or heat sink. The internal support, in the form of a fiberglass mat embedded within the gel composition, is necessary to provide the necessary handleability; otherwise, the gel composition is too soft, tacky, and fragile. However, the support increases the compression modulus of the gel article, so that mechanical stresses are undesirably transmitted to the underlying electronic components.
Thus, it is desirable to develop a method for dissipating heat from bare silicon chip containing circuit boards, while at the same time protecting them from mechanical stresses or avoiding the transmission of such stresses to the bare silicon chips.